Central-venous catheters, that is to say catheters which are inserted into large veins near to the heart, are used for the measurement of given haemodynamic parameters and for the continuous or discontinuous application of medicaments or infusion solutions and are an important component of patient monitoring and (intensive) medical care. The puncturing or aspiration of veins which are near to the heart presupposes skill, experience and knowledge of the anatomical factors involved. Besides the variability of the position of veins near to the heart in particular difficult puncturing conditions repeatedly result in incorrect puncturing of arteries which are in adjacent relationship.
Arterial puncturings or aspirations with thick cannulae as are used for the insertion of guide wires or catheters can result in massive bleedings under pressure into the surrounding tissue for example nerves, and then necessitate operative rectifying intervention. Incorrect arterial puncturing can represent a considerable risk to a patient who must undergo an operative procedure using a heart-and-lung machine and whose blood is made non-coagulatable for that purpose. Puncturing a vessel wall which has been arteriosclerotically altered can result in the mobilisation of plaques which are entrained with the blood flow into smaller arteries and can clog same. For that reason incorrect arterial puncturing in the region of the vessels in the neck which also supply the brain (arteria carotes) can result in a stroke and possibly disability of a patient. Those complications, even if rare, which however are tragic in the individual situation in which they occur and which involve high consequential costs have resulted in the recommendation of firstly finding veins which are close to the heart, by means of a thin needle (pre-puncturing cannula or also referred to as a xe2x80x9cfinder-needlexe2x80x9d) in order then to puncture the vessel with the thicker cannula, in the direction of puncturing of the finder-needle.
In the arteries of the circulatory system of the body a mean pressure of 70-100 mm Hg with an amplitude between systolic and diastolic pressure of about 40-60 mm Hg is physiological, whereas in the veins which are close to the hear of the circulatory system a mean pressure of 7-10 mm Hg with an amplitude of only a few mm Hg is physiological. Under pathological conditions the pressure difference between arteries and veins and also the pressure amplitude in the vessels can become both greater and also smaller.
If an artery is punctured with a cannula having a large lumen, that can generally be recognised by virtue of the spraying and pulsating discharge of blood from the cannula. In contrast after a vein is punctured the blood will flow away from the cannula under a low pressure and fairly continuously Irrespective of whether an artery or a vein has been punctured however the high through-flow resistance of cannulae with a small lumen results in a uniform discharge of blood from the cannula, which at most pulsates discretely and never sprays out. Accordingly recourse must be had to other criteria in order to identify an artery or a vein. The colour of the blood is therefore a poor criterion.
By virtue of the higher oxygen content arterial blood is normally lighter than venous blood. Lung or heart diseases however can have the result that arterial blood also appears comparatively dark: in contrast venous blood is also really light when the patient is breathing with pure oxygen, for example in the context of initiating anaesthesia.
A reliable possible way of distinguishing arterial and venous blood is blood gas analysis in which the oxygen and carbon dioxide partial pressures are measured. That is time-intensive and labour-intensive and thus expensive.
The blood pressure in the punctured vessel can also be measured directly by means of a riser line. When the blood is allowed to flow back into the line the transfer of several milliliters of blood into the line requires some time, depending on the through-flow resistance of the cannula. If in contrast operation is conducted with a riser line which is filled with liquid, for example filled with common salt solution preparation of the riser line is a comparatively time-consuming procedure. At any event an assistant is required in order to hold the riser line.
A pressure-converter system, which is referred to as a transducer system, again detects not only the mean blood pressure as with a riser line but also the blood pressure amplitude in the punctured vessel. As the insertion of catheters presupposes sterile operating conditions, this procedure necessarily means that the transducer system must firstly be prepared in a sterile condition, that is highly time-consuming and also requires a xe2x80x9cthird handxe2x80x9d to which the cable connection for connection to the monitor can be handed over.
In view of the level of expenditure and complication and the difficulties involved in identifying a vein which is close to the heart without any doubt by means of a feeder-needle clinical practice often involves foregoing the pre-puncturing procedure. Unintentional arterial puncturings are then only detected after insertion of the catheter by means of intravasal pressure measurement or by virtue of blood rising in the infusion system and/or by an X-ray of the thorax.
The risk of HIV- (AIDS-) or hepatitis infection by virtue of contact with the blood of infected patients has resulted in xe2x80x9cclosedxe2x80x9d cannula or syringe systems being offered not only for taking blood but also for the insertion of central-venous or arterial catheters. A closed system for example for the pressure-controlled handling of a fluid is known from the present applicant""s DE 195 03 230.
So-called side-port cannulae are in turn available on the market for the low-contamination insertion of central-venous catheters. They have an additional duct which is disposed laterally and which is closed by a valve mechanism and by way of which a guide wire can be introduced after puncturing of the vessel, by way of which wire the catheter is then pushed into the vessel: the so-called xe2x80x9cSeldinger procedurexe2x80x9d. Furthermore, a syringe is also known from U.S. Pat. No. 4,813,938, referred to hereinafter as the xe2x80x9cRaulerson syringexe2x80x9d, with a through duct in the plunger, which is closed by a valve mechanism and through which a guide wire can be introduced into the punctured vessel without the syringe having to be disconnected.
Both systems effectively prevent the escape of blood even after unintentional puncturing of an artery with a cannula having a large lumen, and accordingly give a false sense of security. As the syringe and the cannula do not have to be disconnected, as in the conventional procedure, incorrect arterial puncturing is usually recognised only after insertion of the catheter and thus maximum trauma for the vessel wall.
The object of the present invention is to provide a compact apparatus which is simple to operate and which can measure the pressure in a cavity, and which in particular permits a precise reliable distinction to be drawn between veins and arteries by means of a cannula, wherein the apparatus permits controlled insertion of a catheter or the like, in such a way as to avoid unnecessary injuries, in particular with the exclusion of blood contamination of the person operating the apparatus.
A first configuration of the invention is a one-person-operable, sterilisable, medical pressure measuring apparatus or manometer which has at least one cannula connection, a measuring duct and an entry, and wherein the measuring duct opens into a closed container with rigid wells in which there is a compressible medium.
Therefore, a liquid does not rise against the force of gravity, as in the case of the known construction, in an open container, for example a riser line, but penetrates into a closed container of a given volume and with non-elastic walls, which is for example filled with air, and in so doing successively compresses the air.
The pressure resulting in a closed container of that nature is now calculated from the quotient of the uncompressed volume of air and the volume of air compressed by the liquid which has passed into the container, and is dependent on the specific weight of the entering liquid: if for example 6 ml of uncompressed air is compressed to 5 ml, that gives a pressure of 1.2 bar (=about 910 mm Hg or about 1210 cm head of water) or a pressure above atmospheric of 0.2 bar (=about 150 mm Hg or about 200 cm. head of water).
As is internationally further usual in medical linguistic usage or for reasons of greater clarity reference will be made hereinafter to the pressure units xe2x80x9cbarxe2x80x9d. xe2x80x9cmm Hgxe2x80x9d or xe2x80x9ccm head of waterxe2x80x9d.
By means of that calculation a given pressure can be associated with a given level of liquid in the container and in that way it is possible to calibrate a scale on which the pressure of the compressed air can be directly read off with the column of liquid as a pointer. Having regard to the hydrostatic pressure of the column of liquid which also involves the specific weight of the liquid it is also possible to specify the pressure under which the liquid is. If the liquid is for example xe2x80x9ca physiologicalxe2x80x9d common salt solution with a specific weight of about 1 then the error resulting from a column of liquid of a few centimeters can be disregarded for the measurement of arterial blood pressures.
In a handy pressure measuring apparatus which is also suitable for the measurement of central-venous and arterial mean pressures, connected upstream of the container is the measuring duct which is also of a predetermined volume and also has non-elastic walls for completely receiving the rising liquid, which is either outside or inside the container. In that case, the quotient of the volume of air in the measuring duct to the total volume of air in the measuring duct and the container gives the measuring range over which the pressure can be comparatively accurately read off at the level of liquid in the measuring duct: for example on a measuring duct which accommodates 1 ml of the pressure measuring apparatus with a total air volume of 6 ml, it is possible to read off pressures above atmospheric of between 0 and about 150 mm Hg and thus also the pathologically increased arterial mean pressures of a patient with high blood pressure. A lower ratio between the volume of air in the measuring duct and the total air volume in the measuring duct and container admittedly increases the measuring range but worsens the degree of reading accuracy.
As even when the measuring duct involves a diameter of 3 mm the surface tension of the liquid prevents the liquid passing into the measuring duct due to the force of gravity, the pressure measuring apparatus can be arranged in such a way that the measuring duct extends horizontally and thus the hydrostatic pressure of the column of liquid no longer causes any measurement errors.
Due to the principle involves, such a pressure measuring apparatus measures more accurately in a situation involving only slight compression of the volume of air contained therein, than when a greater degree of compression is involved. The level of measuring accuracy for low pressures can be very easily further improved by means of a measuring duct of different cross-sections (with a cross-section which is smaller close to the cannula and larger towards the container).
The above-described pressure measuring apparatus is now fitted between the cannula and the syringe before the commencement of the puncturing operation. That blood pressure measuring apparatus may however first be xe2x80x9cconnected in parallelxe2x80x9d (=opened) for blood pressure measurement (for example by means of a three-way tap) if blood can be slightly aspirated (=sucked into the syringe cylinder). Otherwise (in spite of the aspiration being xe2x80x9cdampedxe2x80x9d by the volume of air of a xe2x80x9cparallel-connectedxe2x80x9d blood pressure measuring apparatus) air can be sucked out of the blood pressure measuring apparatus into the syringe cylinder, whereby the apparatus loses its calibration and incorrectly high blood pressure values are measured.
The blood pressure can be measured either by blood being allowed to pass under the blood pressure in the punctured vessel into the measuring duct or by a procedure whereby blood is passed under pressure into the measuring duct by injection and then the operator waits for the drop in the column of blood until pressure equalisation occurs. As in both cases only a small volume of blood has to pass into or out of the blood pressure measuring apparatus, in contrast to measuring pressure with a riser line, the blood pressure can be measured very quickly.
The contamination problem is resolved by the cannulae being closed, for example by means of a three-way tap, prior to disconnection of the syringe, and being opened again only for the advance of the guide wire for a catheter or of the catheter itself.
The described pressure measuring apparatus can also advantageously be used on so-called side-port cannulae, as are described above. The attachment of the pressure measuring apparatus only has to be designed in such a way that, by fitting onto the side port, the valve mechanism disposed there is overcome and, after disconnection, closes again (in order then to be able to introduce the guide wire under low-contamination conditions). It is to be observed that some air is generally disposed in the side port of such cannulae, and in the pressure measurement operation that air can result in an incorrectly low pressure indication if that additional volume of air is not taken into consideration when calibrating the scale of the pressure measuring apparatus.
The syringe described in above-mentioned DE 195 03 230, to the disclosure of which reference is directed here in this respect in terms of the full content thereof, is also suitable for blood pressure measurement by means of cannulae having a small lumen. In this so-called xe2x80x9cEnk syringexe2x80x9d the container and the measuring duct in the container are disposed in the plunger of the syringe An advantage of this syringe is that upon venting of the syringe cylinder a pressure measuring apparatus of that syringe is always equalised in relation to ambient air pressure, being what is known as xe2x80x9czeroedxe2x80x9d, and remains calibrated, as the air in the pressure measuring apparatus of the syringe is only in contact with the liquid, which is also at room temperature, for example common salt solution, in the syringe cylinder, and is not directly in contact with blood at body temperature.
The syringe makes it possible for the entire xe2x80x9cmeasuring limbxe2x80x9d to be flushed out of the syringe cylinder by the injection of liquid, for example common salt solution. In that situation liquid is passed into the measuring duct under pressure at the same time. As the friction of the sealing lip always holds the plunger in its position in the syringe cylinder, even when completely relieved of load by the injecting finger, the syringe now injects under decompression of the air in the plunger until the pressure corresponds to the vessel pressure. As in that case the cannula is continually being flushed coagulating blood cannot clog and block the cannula and the vessel wall is always pressed away from the cannula.
The blood pressure can also be measured with such a syringe for example by means of so-called side-port cannulae. The problem of the loss of air from the pressure measuring apparatus of that liquid-filled syringe due to aspiration with the conventional syringe does not arise in that case as the pressure measuring syringe is only connected to the side-port for the blood pressure measurement operation, possibly by way of an adaptor which lifts open the valve mechanism.
A measuring duct of a cross-section which is smaller in the front portion and larger in the rear portion improves the accuracy and readability of the pressure measuring apparatus (=syringe with a low- and high-pressure measuring range): the (mean) pressure of central veins can be accurately measured in the front, long measuring passage portion having a small lumen. Due to the magnifying lens effect of the thick cannula wall the level of liquid or the scale in that region can be clearly seen even if the plunger is pushed into the syringe cylinder. The higher arterial (mean) pressures are measured in the rear, short measuring duct portion having the large lumen, even if such measurement is somewhat less accurate. As that region of the plunger projects out of the syringe cylinder the level of liquid can always be read off here without any problem.
The following calculation by way of example is intended to illustrate this: let there be a plunger with a total air volume of 8 ml: in that case 0.8 ml is apportioned to the measuring duct. If now the measuring duct is completely filled with liquid compression of the volume of air from 8 ml to 7.2 ml corresponds to a pressure above atmospheric of about 0.11 bar (=about 84 mm Hg) with a length of 50 mm and an inside diameter of 2 mm the front measuring duct portion is of a volume of about 0.157 mm corresponding to a measuring range of between 0 and about 0.02 bar (=about 15 mm Hg) pressure above atmospheric. The rear measuring duct portion is of a length of 32.8 mm and is of an inside diameter of 5 mm: that results in a volume of about 0.644 ml and a measuring range of between 0.02 and about 0.11 bar pressure above atmospheric.
In a further configuration the measuring duct can also become continuously larger from the inside diameter.
Another possible way of adapting the pressure measuring apparatus to different pressure measuring ranges is afforded by a closure portion with which the air volume of the plunger, which communicates with the syringe cylinder, can be selectively reduced or increased in size (for example with a closure portion which is displaceable in the plunger and which selectively closes the rear opening of the measuring duct or even parts thereof). In that arrangement a screw mechanism or a bayonet arresting means always holds the closure portion in the defined position in the plunger and thus ensures pressure measuring accuracy.
If a suitable closure or valve mechanism in the closure portion or however the wall of the plunger makes it possible for the cavity in the plunger to be selectively opened for pressure equalisation relative to the ambient air, then a pressure measuring apparatus of that kind can also be used to measure negative pressures on a suitably calibrated scale, for example the pressure below atmospheric which occurs due to aspiration. that is to say by withdrawing the plunger in the syringe cylinder: after the liquid is drawn up into the syringe cylinder the plunger is opened to atmospheric pressure, for example by means of a slider or two openings which are rotatable relative to each other in the wall of the plunger and the closure portion. After some liquid has been urged into the measuring duct as far as a zero marking, for example half of the measuring duct, when the outlet opening of the syringe cylinder is closed, by virtue of a forward displacement movement of the plunger, for example by slowly xe2x80x9cscrewing inxe2x80x9d the plunger by means of a screwthread in the syringe cylinder and on the plunger, the plunger is air-tightly closed off again in relation to atmospheric pressure. Now positive and also negative pressures can be measured. That option is also afforded by a closure portion which is displaceable in the plunger: after liquid has been drawn up into the syringe cylinder and the syringe cylinder has been vented, the closure portion is pulled back or turned back in the plunger until the liquid which is passing into the measuring duct has reached the zero marking.
As a particularly desirable configuration of a pressure measuring apparatus. described hereinafter is a system having a syringe with which it is not only possible to implement an aspiration procedure, but also the pressure in the punctured vessel or liquid cavity can be measured and through which a guide wire for a catheter or a catheter itself can subsequently be pushed for example into a blood vessel, without the syringe having to be disconnected from the cannula.
The syringe has a syringe cylinder and a hollow plunger with a damping and pressure measuring device corresponding to the xe2x80x9cEnk syringexe2x80x9d. At the rear the plunger has a closure portion with a bore therethrough, which can be opened or closed by a sliding or rotating mechanism or a valve mechanism which is known for example from the xe2x80x9cRaulerson syringexe2x80x9d, and through which for example a guide wire for a catheter or a catheter itself can be moved forwardly into the duct of the pressure measuring device. If the duct additionally has at least one sufficiently large lateral opening the duct of the pressure measuring device can also be accommodated by the closure portion in the sense of a centering effect.
The handling of that syringe is described hereinafter by reference to the example of puncturing a blood vessel and inserting a guide wire for a catheter into the vessel:
By retraction of the plunger, a pressure below atmospheric which is xe2x80x9cdampedxe2x80x9d in comparison with a normal syringe also occurs in the syringe cylinder of this syringe: liquid, for example common salt solution can be drawn up or raised into the syringe cylinder by the reduced pressure, against the force of gravity. In that procedure the syringe must be held with its cannula attachment downwardly and there should be some air in the syringe cylinder, about 1 ml. Otherwise liquid can penetrate into the duct due to the pressure below atmospheric which also obtains in the plunger, particularly in the case of a strong aspiration effect. A bar which is disposed at the end of the plunger projecting out of the syringe cylinder and which fits into a corresponding opening in the syringe cylinder only in a given rotational position ensures that the plunger cannot be accidentally entirely pushed into the syringe cylinder.
The syringe is now vented conventionally, that is to say held with the cannula attachment up, and the air is dispelled from the syringe cylinder by moving the plunger forwardly. When that happens the liquid closes the front opening of the duct. Even with a duct opening of three millimeters the surface tension of the liquid prevents liquid from penetrating into the duct due to the force of gravity. When venting the syringe the air in the plunger is closed off from the surrounding atmosphere and thereby the pressure measuring device is xe2x80x9czeroedxe2x80x9d (=calibrated to zero).
In the attempt to puncture a blood vessel with the cannula fitted onto the syringe, it is never possible to aspirate as extremely as with a conventional syringe as the air in the plunger also xe2x80x9cdampsxe2x80x9d the aspiration effect. This can prevent punctured veins from collapsing or the cannula from being sucked against the vessel wall, as is otherwise possible in she event of a strong aspiration effect. As no blood is to be aspirated in those situations, it is erroneously assumed that the blood vessel has not yet been reached or has already been left again. That causes an increase in the number of incorrect or multiple puncturings. Accordingly a xe2x80x9cdampedxe2x80x9d aspiration effect can certainly be advantageous.
If a blood vessel is punctured, as can be seen from the sudden entry of blood into the syringe cylinder, the liquid in the syringe cylinder acts as a highly effective buffer. Even in the event of a strong aspiration effect, that prevents the blood-liquid mix from being sucked directly into the duct, and thus prevents the pressure measuring device from losing its calibration. In addition the blood is immediately diluted by the liquid and thus cannot coagulate in the syringe. Admittedly a slight increase in temperature and thus expansion of the air also occurs, with the passage of time, by virtue of mixing of the liquid at ambient temperature with the blood which is at body temperature. As however the blood pressure is now measured directly, the resulting measurement error (wrongly low values) is also negligible having regard to the pressure difference to be detected as between veins and arteries.
For blood pressure measurement in the punctured vessel, the plunger is now advanced somewhat. As a result the blood-liquid mix is partially injected and partially urged into the duct. As the plunger always retains its position in the syringe cylinder due to a sufficiently high level of friction of the sealing lip, even if the finger which is advancing the plunger completely relieves the load thereon, the blood pressure of the punctured vessel can be read off, for example at a calibrated scale in or on the plunger, as soon as the level of liquid in the duct is no longer moving.
The xe2x80x9ccolouredxe2x80x9d column of the blood-liquid mix as a xe2x80x9cpointerxe2x80x9d of the pressure measuring device is easily visible. However even a column of clear liquid can be clearly seen in the duct by virtue of neutralisation of the birefringence effect of the duct wall.
Alternatively it is also possible to operate with a syringe which is prepared for the measurement of negative pressures (see above): after the puncturing cannula has been inserted the plunger is withdrawn in the syringe cylinder until the desired pressure below atmospheric is attained. With a suitable design in respect of the circular sealing lip of the plunger, the plunger is in that case again held in its position in the syringe cylinder and thus the set reduced pressure is maintained. When the cannula punctures a blood vessel, the level of liquid in the duct rises abruptly until pressure equalisation with the pressure in the vessel occurs.
If measurement of the blood pressure shows they the desired vessel, for example a vein, has been punctured, the plunger is pushed completely into the syringe cylinder and thereafter the guide wire for the catheter or the catheter Itself is pushed into the blood vessel through the closure portion, the measuring duct, the cannula attachment of the syringe cylinder and the cannula fitted thereon. In that situation the blood-liquid mix is injected. If by mistake the measuring duct is xe2x80x9cover-injectedxe2x80x9d due to very rapid injection, the blood-liquid mix drips into the cavity of the plunger and is xe2x80x9cenclosedxe2x80x9d there. Contamination of the working area is avoided in that way. As however an increased pressure can thus be built up in the plunger, pass due to the air out of the plunger into the syringe cylinder and be injected due to inattentiveness. xe2x80x9cover-injectionxe2x80x9d of the duct should be avoided.
Injection of the blood-liquid mixture does not represent a danger from the point of view of the patient: admittedly, activation of coagulation of the aspirated blood may occur due to the contact with a foreign surface. However the coagulation factors are immediately severely diluted by the liquid in the syringe cylinder. The blood cannot therefore xe2x80x9cclotxe2x80x9d and a blood clot will never be injected.
In principle it is necessary to operate with such a syringe xe2x80x9cfrom above downwardlyxe2x80x9d, that is to say the cannula attachment on the syringe is the lowest point and the end of the plunger is the highest. That corresponds to the working situation when puncturing the vena jugularis irterna, which is the central-venous entry that is in fact most frequently used, and the vena femoralis. Admittedly, in an aspiration situation air passes out of the plunger into the syringe cylinder but that air is xe2x80x9ccapturedxe2x80x9d again in quantitative terms after puncturing of a blood vessel with the penetration of liquid into the syringe cylinder or in the forward movement of the plunger. If in contrast operation is effected xe2x80x9cfrom below upwardlyxe2x80x9d air is permanently lost from the plunger and there is the danger of mistakenly injecting the air which has passed into the syringe cylinder, in that case also liquid is sucked into the measuring duct due to the reducer pressure in the plunger and the pressure measuring device is mis-calibrated as a result.
Even when working correctly a small air bubble remains in the angle between the syringe cylinder and the front face of the plunger. The mis-calibration (wrongly high pressure indication) resulting from that remaining loss of air from the pressure measuring device into the injection cylinder becomes progressively smaller with the same lost volume with an increasing air volume in the plunger. As when inserting catheters into blood vessels it is the difference between venous and arterial pressure hat is being considered, and thus a pressure difference of at least 50 mm Hg or about 65 cm water head, that error is scarcely relevant. If the procedure adopted is such that the cannula fitted onto the cannula attachment is not vented prior to the puncturing operation, then the air in the cannula completely compensates for the calibration error if the volume corresponds to the loss from the plunger.
The air which has passed into the syringe cylinder upon aspiration can also be completely xe2x80x9ccapturedxe2x80x9d again by means of a plunger front face which is shaped concavely towards the opening of the measuring duct if, as described above, operation is implemented with the syringe xe2x80x9cfrom above downwardlyxe2x80x9d. In that case however the cannula must be vented, otherwise the air in the cannula can again cause a small calibration error (wrongly low pressure indication).
If now the measuring duct is arranged not centrally but eccentrically in the plunger, the measuring duct can be closed prior to aspiration by rotation of the plunger relative to the sealing lip. The plunger is first rotated again for pressure measurement and for subsequent insertion of a guide wire or catheter, in such a way that a opening in the sealing lip comes into alignment with the measuring duct.
The sealing lip can be easily prevented from also rotating by virtue of a thickening in the wall, for example a bar, at the inside wall of the injection cylinder and a corresponding opening in the sealing lip. The thickening in the wall which acts like a magnifying lens improves the perceptibility of the pressure indication which does not have to be read off through three layers of plastic material, as in the case of a measuring duct which is disposed centrally in the plunger, but possibly only through two layers of plastic material.
By virtue of two mutually oppositely disposed spherical thickenings of the sealing lip the correct rotational position of the plunger for closure or opening respectively of the pressure measuring device can be easily detected: the double seal which xe2x80x9clatchesxe2x80x9d into the lumen of the measuring duct reliably closes off the air volume in the plunger upon aspiration, relative to the exterior. The air of the pressure measuring device cannot in that way escape from the plunger even in the event of strong aspiration which in this case is xe2x80x9cundampedxe2x80x9d.
The rotational position, which is necessary for pressure measurement or for advance movement of the guide wire or catheter and which also involves a xe2x80x9clatchingxe2x80x9d effect, of the plunger relative to the sealing lip and relative to the cannula attachment of the syringe cylinder, which is also disposed in eccentric relationship, is in contrast afforded when a mark on the plunger is brought into alignment with a mark on the syringe cylinder and thus for example the eccentrically disposed measuring duct is directly under he thickening of the wall of the syringe cylinder.
Air-tight closure of the pressure measuring device can also be achieved by means of a circular sealing lip of variable depth, which lies in a circular groove in the plunger of also variable depth: depending on the rotational position of the plunger the sealing lip is squashed together to a greater or lesser degree and thereby an opening in the sealing lip is selectively opened or closed. The friction of the sealing lip can also be varied in that way as desired.
A comparable mechanism in the closure portion of the plunger serves to open the plunger after pressure measurement has been effected for introducing the guide wire or catheter.
If the measuring duct does not have at least one large, laterally disposed opening to the cavity of the plunger, the bore in the closure portion should not form a through communication with the measuring duct: on the one hand a gap between the measuring duct and the closure portion does not cause a problem in the forward movement of the guide wire or the catheter. On the other hand it is crucial for correct functioning of the pressure measuring device and like a laterally disposed opening ensures that even after arterial puncturing upon opening of the bore in the closure portion blood can at most drip out of the measuring duct into the cavity of the plunger. In the case of a bore which passes through the entire plunger in contrast only a valve mechanism which is more complicated from the point of view of structure and production procedure and which is thus more expensive prevents blood from issuing from the syringe.
With that syringe it is not absolutely necessary to operate xe2x80x9cfrom above downwardlyxe2x80x9d: as the pressure measuring device in the plunger is closed after preparation of the syringe (drawing up liquid, venting the syringe) and is opened for the pressure measurement procedure only after puncturing of a blood vessel has been effected, no air is xe2x80x9clostxe2x80x9d from the piston in the aspiration operation and therefore also does not need to be xe2x80x9ccapturedxe2x80x9d again.
In contrast to the above-described configurations of the invention it is therefore possible with this syringe also to operate xe2x80x9cfrom below upwardlyxe2x80x9d, as is often necessary in operations for puncturing the vena subclavia. In addition aspiration is effected with this syringe as with a conventional syringe in an xe2x80x9cundampedxe2x80x9d mode.
After the correct cannula position has been secured by means of blood pressure measurement the plunger is pushed entirely into the syringe cylinder again in order subsequently to be able to advance the guide wire or catheter through the syringe into the blood vessel. If in that situation there is a wish to avoid completely injecting the blood-liquid mix, the bore in the closure portion can already be opened prior to the forward movement of the plunger: the blood-liquid mix is now partially injected into the cavity of the plunger. When that happens the air escapes from the plunger through the bore in the closure portion. Accordingly a pressure above atmospheric does not build up in the plunger and air cannot therefore possibly pass into the syringe cylinder.
A further possible way of switching off the pressure measuring device of the syringe in the aspiration operation is afforded by a mandrel or bar portion which sealingly closes the front opening of the measuring duct. That bar portion is only withdrawn from the measuring duct when pressure is to be measured or inserted again when aspiration is to be implemented once more. This construction requires a valve mechanism in the closure portion. In this construction the front opening of the measuring duct should be of a smaller cross-section than the measuring duct itself, otherwise the bar portion in the measuring duct can behave like a plunger and upon being withdrawn suck liquid into the measuring duct.
Withdrawal of the bar portion results in a calibration error: the volume of the bar portion is suddenly missing and causes a minimal pressure below atmospheric in the pressure measuring device, with the consequence of a wrongly high pressure indication. If the bar portion is of a suitably small volume however the calibration error can be disregarded or can be taken into consideration when calibrating the scale from the outset.
If the bar portion is already completely pulled out of the plunger for the pressure measurement operation, as is necessary for advancing the guide wire or catheter, the calibration error can also be minimised in the following way, if the valve mechanism in the closure portion of the plunger is of a suitable design configuration: if when the bar portion is pulled out the function of the valve mechanism is neutralised for a brief moment, the pressure of the air in the pressure measuring device becomes equal to the ambient air pressure (xe2x80x9czeroingxe2x80x9d) It is to be noted in that respect that, if the valve function is neutralised for an excessively long period of time, due to the blood-liquid mix penetrating into the measuring duct, too much air can also escape from the pressure measuring device and the calibration error is increased in that way (incorrectly high pressure indication). This variant of the syringe can also be operated. xe2x80x9cfrom below upwardlyxe2x80x9d and aspiration can also be effected in an xe2x80x9cundampedxe2x80x9d manner.
In the case of a syringe which is also prepared for the measurement of negative pressures and in which the pressure measuring device is for example correctly xe2x80x9czeroedxe2x80x9d with a half-filled measuring duct, the problem of a loss of air from the pressure measuring device into the syringe cylinder occurs only to a limited degree. As long as aspiration is not effected to a greater extent than is detected by the measuring range of the syringe, no air is xe2x80x9clostxe2x80x9d.
A problem which in principle is greater, in regard to mis-calibration of the pressure measuring device, arises in connection with sucking in air in the case of a cannula which is not air-tightly fitted onto the cannula attachment (incorrectly low pressure indication). Due to the principle involved in such a case the syringe has to be vented afresh on each occasion.
Further advantageous configurations and developments and features of the invention are described in greater detail in the following specific description. In the drawing: